The present disclosure relates to the use of swellable polymers in downhole operations.
Wellbore fluids used in oil and gas exploration and production use a variety of additives to achieve a desired property for the fluid or to produce a desired result in the wellbore. One example of an additive that can serve many purposes is a water-swellable material. For example, in a swollen form, these materials can increase the solid/liquid volume ratio of a wellbore fluid, which, when placed in a permeable portion of the formation, may allow for the swollen materials to plug or reduce fluid flow through that permeable portion of the formation resulting in problems such as lost circulation of wellbore treatment fluids.
Generally, the swellable polymers are placed downhole at the permeable zone by mixing with a carrier fluid and introducing the fluid downhole. However, such techniques may, in some instances, limit the concentration of the swellable polymers in the carrier fluid because as the polymer swells, the fluidity or pumpability of the fluid decreases. Because many swellable polymers can increase in volume by about 400%, the pumpability of the fluid may decrease prematurely even with relatively low concentrations of swellable polymer and interfere with placement in the correct location, and in some cases may reduce the depth of placement, for example in a fracture. In such cases use of large quantities of carrier fluid may have to be used to facilitate pumping, which can be time consuming and costly. In cases, where placement deep inside a fracture or cavernous zones becomes problematic due to swollen particle sizes, carrier fluids with high salt concentrations are used in which the particle swelling is less. Once the partially swollen particles are placed within a zone as desired, a subsequent fluid based on fresh water or low-salt concentration brine, is pumped through the swollen particle mass to increase the swollen particle volume in situ. The decrease in swollen volume of water-swellable particles by salt solutions is dependent on the type of salt and concentration. It is a common practice to use monovalent salts such as sodium chloride or potassium chloride. The swellable-particle volume increase with monovalent salts is still significantly high, and may be subject to the same limitations as fresh water systems.
The use of fluids containing divalent ions further decreases the swelling of the particles, which allows for ease of pumping and deeper placement of particles inside a high permeability zone, such as a fracture. However, further swelling of the particles in situ upon pumping fresh water or an aqueous fluid containing lower salt concentration is negligible. Therefore, plugging of such zones by swellable particles is not practical. Thus, there is a need for fluid compositions containing swellable particles that allow for less swelling during placement and enhanced swelling in situ after placement.